Gas turbine engines have been used for decades for propulsion, power generation and other industrial applications. A gas turbine engine extracts energy from the flow of combustion gas. It has an upstream compressor coupled to a downstream turbine with a combustion chamber positioned between. Energy is added to the stream of compressed air in the combustor where fuel is mixed with the air and ignited. This combustion increases the temperature, velocity and volume of gas. The hot gas is diverted through a stationary nozzle that is then deflected onto turbine blades, spinning the turbine rotor and powering the compressor. Additional stages of vanes and blades are used to produce more work. Energy is extracted in the form of shaft power or thrust. The thermal efficiency and power output of these engines increases with increased turbine rotor inlet temperature, up to the hydrocarbon fuel stoichiometric limit of about 4,200 degrees Fahrenheit. With the melting temperature of the nickel based super alloys used to fabricate components for gas turbines at about 2,000 degrees Fahrenheit, it should be evident that cooling a gas turbine component is critical to its sustained operation. In the case of a gas turbine blade, combinations of thermal barrier coatings and sophisticated cooling techniques have been developed to maintain the blade's temperature at a safe operating point.
Referring to FIG. 1, as with other components in a gas turbine engine, a turbine blade (10) is formed as a hollow airfoil that is cooled both internally and externally. The coolant used for this purpose is extracted from the compressor, resulting in a reduction in the thermal efficiency for the engine. As such, the amount of coolant extracted is minimized by design and its mass becomes a major design characteristic for the turbine blade. Internal cooling is accomplished by injecting the coolant through an opening (7) in the bottom of the blade and allowed to flow through a series of serpentine passages inside the blade, where heat is extracted from the inner airfoil surface. In addition, jet impingement, turbulator and pin fin cooling are used to further extract heat from the inner airfoil.
External cooling, known in the industry as film cooling, injects a coolant onto the outer airfoil surface at desired locations along the airfoil. Film cooling features (12) on the leading edge of airfoil, the region of the airfoil that has the highest heat transfer rate, are known in the industry as showerhead holes (11). Cooling features on the concave and convex sides of the airfoil are known as pressure (15) and suction (13) holes respectively. Cooling features on the trailing edge (17) of the airfoil are usually known as trailing edge slots. Cooling features on the tip (14) of the airfoil are known as either tip or squealer holes depending upon their location. Film cooling protects the blade's airfoil surface directly at the immediate and downstream injection region, as opposed to internal cooling techniques. To a lesser degree, film cooling provides additional heat removal from the airfoil by convection as the coolant flows through the wall of the airfoil.
Referring to FIG. 3, it is the size, shape and location of the immediate injection region that is named the film cooling effect (40). One should recognize that if the film cooling effect is too small, a result of either a reduction in the mass of coolant flowing through the cooling feature or defective cooling feature geometry, the blade's lifespan is decreased. Furthermore, if the film cooling effect is severely out of location, the blade's life span will be decreased. As in the case of an aircraft engine, this decreased lifespan could result in the loss of life. Therefore, the size, shape, and location of the cooling effect are critical design characteristics for the turbine blade. One should readily recognize from this discussion that the design intent of cooling features is the film cooling effect, and not the incidental characteristics such as its geometry, location and mass flow rate.
The prior art of methods for measuring a film cooling effect vary greatly between the environments of research and development, and manufacturing of gas turbine components. Research and development methods are distinguished by their enormous instrumentation, operational costs and considerable amount of time needed to accomplish a measurement. Manufacturing methods are characterized as being cheaper and quicker, but do not directly measure the film cooling effect and rarely measure every individual film cooling feature which would require isolation from the remaining plurality of features.
Research and development methods use designed experiments on actual components or simplistic models. The experiments are designed to measure the heat transfer coefficient, mass transfer analogy or film effectiveness of cooling features. Heat flow gauges, thin foil heaters with thermocouples, copper plate heaters with thermocouples, naphthalene sublimation, foreign gas concentration sampling, swollen polymer, ammonia diazo, pressure sensitive paint, infrared thermography, thermographic phosphors, liquid crystal thermography, hot and cold wire anemometry, laser doppler velocimetry, particle image velocimetry, laser holographic interferometer and surface visualization are some of the most common used in the industry. The cost and time associated with using these methods prohibit their use in a manufacturing environment.
Known manufacturing methods infer measurement of a film cooling effect by a combination of measurements. For example, measurement of the dimensional geometry and location of the cooling feature is combined with the measurement of the mass rate of air flowing through the cooling feature. Modern film cooling features are designed to have compound angles and complex shapes, complicating dimensional measurements. Turbine blades in particular may need hundreds of cooling features, complicating the flow measurement of an individual feature. As such groups of features are isolated and the collective mass rate of airflow measured.
All of these manufacturing methods are repetitive, time-consuming and rely on human intervention. Regardless of the manufacturing method used, the film cooling effect is never directly measured, but inferred from the combination of incidental measurements.
As can be recognized, there is the need for a new method that can automatically measure a film cooling effect faster, more precisely, and less expensively than known methods. Embodiments of the invention herein described solve these and other limitations in the prior art.